This invention relates generally to a pressure and temperature (P & T) responsive valve and, more particularly, to a low cost P & T valve having a molded plastic body.
P & T valves are used extensively with water heaters to prevent a build-up of excessive temperature or pressure. If pressure in a typical water heater exceeds a certain level it will cause a seat disc of a P & T valve to lift away from a seat in the valve body against the force of a bias spring. Pressure in the water then is relieved through the seat disc to atmosphere. The force generated by the bias spring is usually adjusted by means of a sealed and threaded cap or by a press-in stamping In addition, the same seat disc can be displaced from the seat in the body by a temperature responsive wax motor element The wax motor element has a non-linear movement so that an output stem achieves most of its travel through a relatively narrow temperature range. A proper mixture of waxes and the initial position of the element insures that the element opens the valve disc at the proper temperature. Generally, the wax motor element is located in close proximity to the seat by means of a machined support. The support must have space around it and between it and the valve seat so there can be sufficient flow to relieve an over temperature or an over pressure condition in the water heater. This requirement for flow around the element support dictates the construction technique used for the valve body. Typically, valve bodies are sand cast to provide the coring that allows for adequate space and flow around the support. Such sand casting contributes significantly to the ultimate cost of the valve. A P & T valve could be made for less cost if plastic molding techniques could be used for the valve body. However, a plastic molded body would introduce problems associated with dimensional stability, and flow.
The object of this invention, therefore, is to provide an improved, lower cost P & T valve.